1. Field of the Invention
The present invention relates to a heater system, and a method of operation thereof, which utilizes a two conductor element, and, more particularly, to an electric heating blanket which incorporates a single conductor heating element and a single conductor sensing element.
2. Description of the Related Art
Blankets are woven from a variety of materials including wool, cotton, synthetics, and various animal and vegetable fibers. Blankets are used as a shawls, bed coverings and horse coverings. The blanket making of primitive people is one of the finest remaining examples of early domestic artwork. For example, the blankets of Mysore, India, are famous for their fine, soft texture. The loom of the Native American, though simple in construction, can produce blankets so closely woven as to be waterproof. The Navaho, Zuxc3x1i, Hopi, and other Southwestern Native Americans are noted for their distinctive, firmly woven blankets. The Navahos produced beautifully designed blankets characterized by geometrical designs woven with yarns colored with vegetable dyes. The ceremonial Chilcat blanket of the Tlingit of the Northwest, is generally woven with a warp of cedar bark and wool and a weft of goats"" hair. Blankets, like society, have changed significantly over the years and it was in the 20th century that the electric blanket, with electric wiring between layers of fabric, gained wide popularity.
The direct conversion of electric energy into heat was first described by the English physicist James P. Joule. According to Joule""s law, a conductor carrying a current generates heat at a rate proportional to the product of the resistance of the conductor and the square of the current. It is the use of this principle, of applying electrical energy to a distributed resistance incorporated in a blanket, which provides warmth to the user. Joule""s law also points to a potential problem, if resistance is locally increased, in a distributed resistive element, more heat is produced in that localized area causing a local hot spot.
In spite of the advantages to the users of electric blankets, consumers have voiced concerns. Manufactures of electric blankets have addressed consumer concerns with scientific studies, incorporation of safety features and marketing techniques. Regardless of the manufacturer""s care in manufacturing electric blankets, localized hot spots can occur in an electric blanket as a result of either a manufacturing defect, handling damage or consumer misuse. A localized hot spot in an electric blanket may cause damage to the electric blanket, property loss and/or injury to the user.
U.S. Pat. No. 5,861,610 (Weiss) discloses a heater wire with a first conductor for heat generation and a second conductor for sensing the temperature. The second conductor is made of 99.5% nickel. The first conductor and second conductor are each wound as coaxial spirals with an insulating material isolating the two conductors. A problem with the Weiss invention is that winding two coaxial spirals separated by an insulating material is rather expensive.
What is needed in the art is an electric blanket with a two conductor heating element which is economical to manufacture, is safe and provides disconnection if a localized heating problem occurs therein.
The present invention provides a two wire heating element. One wire is electrically resistive for the production of heat and the second wire is utilized for the sensing of the temperature of the assembly.
The invention comprises, in one form thereof, a resistive heating conductor having at least one strand, a first electrical insulation surrounding the resistive heating element, a sensor conductor helically disposed around the first electrical insulation and a second electrical insulation surrounding both the sensor conductor and the first electrical insulation.
An advantage of the present invention is that if the heating conductor overheats, electrical power thereto is removed.
Another advantage is that localized overheating of a resistive conductor is detected.
Yet another advantage is that the temperature at which a localized heating problem is detected is predetermined by the selection of the melting temperature of an electrical insulation.
A further advantage is that the electrical heating element is easily manufactured as a stranded resistive conductor with a sensor conductor helically wound around an insulation covering the stranded heating resistive conductor.